This invention relates to microwave coupler devices and particularly to such devices employing microstrip construction.
The microwave coupler known as the hybrid circuit is a four-port device having a certain characteristic impedance at its input and output ports and in which microwave energy supplied at one of its ports is split evenly and produced as output at two of its ports (in a desired in-phase or out-of-phase relation); these two ports are connected to matched load impedances, and no energy is supplied to a matched load at the fourth terminal. Such couplers are components of many more complex devices used widely in microwave systems of various types, especially where a tight or 3-db coupling is achieved. Where such couplers are constructed employing modern microstrip construction in which metallic depositions and etched circuits are used to fabricate extremely small circuit devices, the dimensions and tolerances of the various elements of such couplers become extremely small and close. The difficulty of construction is made that much greater where tight or 3-db coupling is employed.
In designing and fabricating such microstrip couplers for various applications, two overall parameters of concern are that of the overall characteristics impedance of the coupler as seen by a source of microwave energy or a load, to ensure appropriate matching and that of the degree of coupling between the different ports of the coupler device. In determining these parameters, they are generally formulated as functions of two other parameters, namely, the impedances associated respectively with the even-mode and the odd-mode of TEM transmission. The characteristic impedance (input and output) of the coupler is a function of the product of the even-mode and odd-mode impedances. The degree of coupling is a function of the ratio of the even-mode and odd-mode impedances. It is often very difficult to compute these functions and to determine the various dimensions such as the width of the microstrip conductors and the height or spacing between those conductors and the ground plane, as well as the spacing between the conductors themselves. These dimensions may be in terms of mils (thousandths of an inch) or fractions of mils, and correspondingly the tolerances of such small dimensions become extremely tight. It thus becomes extremely difficult both to design and to fabricate such couplers in the modern microstrip technology. Modes of construction of coupler devices dealing with these problems are described in the U.S. Pat. Nos. to Cohn, 3,237,130, Clar, 3,512,110 and Gerst, 3,568,098.
It is among the objects of this invention to provide a new and improved hybrid coupler.
Another object is to provide a new and improved microwave coupler of the microstrip type. Another object is to provide a new and improved microstrip coupler which can be reliably and readily designed. Another object is to provide a new and improved microstrip coupler which can be accurately and reliably fabricated.